1. Field of the Invention
The present invention relates to a method for producing high purity nickel sulfate which can be utilized in a field in which it is intended to obtain high purity nickel sulfate that can be used in a battery material with less impurities, particularly less magnesium, manganese and calcium, from an acidic solution containing nickel.
2. Description of the Related Art
Nickel is widely used as a material for stainless steel or corrosion-resistant alloys, and recently, nickel is also used to a large extent as a material for nickel hydrogen batteries or lithium ion batteries that are used in hybrid electric cars, mobile telephones, personal computers, and the like.
Nickel used in such a material is produced by mining mineral ores that exist in the form of sulfide ore or oxide ore, and smelting the mineral ores.
For example, as a method applicable in the case of treating sulfide ores, the ores are introduced into a furnace to melt, impurities are separated therefrom as slag, and thereby a matte of concentrated nickel is obtained. This matte is dissolved in sulfuric acid or hydrochloric acid, impurities are separated from the dissolved solution to obtain a nickel solution, and a nickel salt such as nickel sulfate or nickel oxide is produced by techniques such as neutralization and crystallization. Alternatively, nickel metal may be produced by performing electrolytic winning or the like.
On the other hand, as a method applicable in the case of treating oxide ores, for example, a process of heating and melting the ores together with a reducing agent such as cokes so as to separate the metal from slag, and obtaining ferronickel, which is an alloy of nickel and iron, to be used as a raw material of stainless steel, has been carried out.
However, such smelting methods all require large amounts of energy, and large cost and extensive efforts are required for the separation of impurities.
Particularly, in recent years, high quality ores have been depleted, and securement thereof is no longer easy. As a result, the nickel level in the available ores tends to be lowered, and thus more cost and efforts are required now to obtain nickel from these low level raw materials.
Thus, recently, a method of obtaining a nickel salt or nickel metal by subjecting low level oxide ores that have not been traditionally used as raw materials, to acid leaching at high temperature under pressure, and neutralizing the leached solution with an alkali such as slaked lime, has been developed.
This method is a technology capable of effectively utilizing low level resources with effective and relatively less energy; however, when it is intended to obtain nickel salts such as described above, new problems not found in conventional smelting methods are emerging.
For example, magnesium, manganese, and the like that are contained in ores are mostly distributed to the slag in the smelting method of using a furnace as described above, and the metals are distributed less to the matte. As a result, the amount of incorporation thereof into nickel salts was limited to a very low level, and the incorporation hardly caused any problem.
On the contrary, in a smelting method using high temperature pressure leaching, magnesium or manganese is thoroughly leached by acid, and consequently, incorporation of the elements into nickel salts is also increased. Furthermore, in the high temperature pressure leaching, an operation of adding an alkali to the leached slurry thus obtained to adjust the pH is carried out, but the influence of the incorporation of calcium that is used as a neutralizing agent to the nickel salts cannot be neglected.
Particularly, when nickel is used in a material for lithium ion batteries or nickel hydrogen batteries, if magnesium, calcium, or chloride ions are co-present, these ions largely affect the characteristics of a battery obtained as a final product. Therefore, high purity nickel salts in which the incorporation is excluded as much as possible from the stage of producing the nickel salts, are considered desirable.
However, in order to obtain nickel sulfate, which is one of nickel salts, with high purity, for example, a method of crystallizing nickel sulfate, such as by obtaining nickel first as metal according to a method such as electrolytic winning, dissolving this metal again in sulfuric acid, and then concentrating the dissolved liquid, may also be considered. However, in order to obtain metal, significant electric power and facilities of a corresponding scale are needed, so that when energy efficiency or cost is taken into consideration, the method is not an advantageous method.
Furthermore, ores containing nickel also simultaneously contain cobalt in many cases. Cobalt is also a valuable metal, and it is not necessary for cobalt to co-exist with nickel. Therefore, the metals are separated and respectively collected.
As an efficient and practical method for separating nickel and cobalt in a sulfuric acid solution, solvent extraction is used in many cases. For example, Japanese Unexamined Patent Publication No. 10-310437 discloses an example of separating nickel and cobalt by extracting cobalt by solvent extraction using trade name: PC88A (manufactured by Daihachi Chemical Industry Co., Ltd.) as an extractant.
When PC88A is used as this extractant, the behavior of extraction of magnesium or calcium is also similar to the behavior of nickel. Therefore, when a solution containing nickel at a high concentration is submitted to solvent extraction, there occurs a problem that the efficiency of separating magnesium or calcium is decreased, such as that the extraction efficiency of magnesium or calcium is decreased.
On the other hand, Japanese Unexamined Patent Publication No. 10-30135 discloses a method for producing an aqueous solution of high purity nickel which does not include any sodium or ammonia, from an aqueous nickel solution containing calcium, magnesium, cobalt and the like as impurities, by extracting and separating impurities in the aqueous nickel solution using an alkylphosphonic acid ester or an alkylphosphinic acid, each containing nickel, as an extractant.
According to the method suggested in Japanese Unexamined Patent Publication No. 10-30135, in which nickel is extracted in advance into an organic solvent at a high pH, and this organic solvent containing the extracted nickel is brought into contact with the nickel solution containing impurities, there occurs an exchange reaction by which elements that are more likely to be extracted than nickel are transferred to an organic phase, while nickel in the organic solvent is transferred to an aqueous phase, and impurities in the nickel solution can be removed.
Furthermore, this is also effective as a method for preventing impurity elements such as sodium that are included in a pH adjusting agent, from being incorporated into the nickel solution and contaminating the product.
However, even in a liquid washing step for nickel sulfate as suggested in Japanese Unexamined Patent Publication No. 10-30135, magnesium in the solution exhibits a behavior similar to that of nickel, and it has been difficult to remove magnesium.
Furthermore, in a case in which impurities such as iron or aluminum are included in a large amount in a nickel-containing material that serves as a raw material, if it is intended to separate these by methods such as neutralization, a large amount of neutralizing agents is required. Further, when impurities are precipitated, there is a possibility that valuable materials such as nickel and cobalt may also be co-precipitated and be lost. Thus, it has not been easy to perform an efficient operation.
Due to such reasons, there has been a demand for a practical method in which high purity nickel sulfate that has a low level of magnesium or chloride and can be used in a battery raw material can be efficiently obtained from a sulfuric acid acidic solution containing a large amount of metal ions of magnesium or the like, or chloride ions.
In view of such circumstances, an object of the present invention is to provide a method for producing high purity nickel sulfate having a low level of impurities, particularly a low level of metal ions such as magnesium or chloride, by adjusting the concentration of an extractant and the pH concentration at the time of treatment in a step of obtaining a nickel sulfate solution with a high nickel concentration by solvent extraction using an acidic organic extractant.